To date, efforts to laser cool Alkaline Earth atoms have focused nearly exclusively on the even atomic number isotopes wherein a lack of ground-state magnetic sub-structure excludes sub-Doppler cooling. Fermionic Alkaline Earth isotopes, such as 87Sr, however, possess nonzero nuclear magnetic moments (for 87Sr, I=9/2) that create both ground and excited-state magnetic sub-structure. The presence of ground-state magnetic degeneracy brings an interesting prospect of direct sub-Doppler cooling of 87Sr on the strong 1S01P1 transition, potentially enabling single-step cooling of large atom numbers to ultra-low temperatures. Moreover, the ground-state magnetic sensitivity, with an origin in nuclear spin, is 1000-times smaller than for electronically originated structures. Thus, sub-Doppler cooling is expected to be minimally affected by MOT magnetic fields. The unique 87Sr excited-state hyperfine structure also presents an excellent testing ground for multi-level sub-Doppler cooling theories [1]. Specifically, although systems such as 87Sr that have significant spectral overlap in the excited-state manifold are expected to display sub-Doppler temperatures, this behavior has so far not been observed [2]. Our systematic study of 87Sr sub-Doppler cooling thus provides critical insight into the cooling dynamics of these complex systems.

© 2003 Optical Society of America

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